Viscoelastic disc clamp using adhesive with radial compliance

ABSTRACT

Herein is disclosed a method and apparatus for coupling a disc to a hub, so as to dampen radial vibrations of the disc. A hub rotates and is coupled to a magnetically encodable disc via a flexible joint. The flexible joint may be made out of a viscoelatic adhesive. There exists no rigid coupling between the disc and the hub, meaning that radial vibrations of the hub result in compression or decompression of the flexible joint, rather than in translation of the disc. The disc may be coupled to the hub by applying a viscoelastic adhesive to a hub, and applying a disc to the viscoelastic adhesive without otherwise rigidly coupling the disc to the hub.

RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of U.S.Provisional Patent Application Serial No. 60/249,006 filed Nov. 15, 2000and entitled “VISCOELASTIC DISK CLAMP USING ADHESIVE WITH RADIALCOMPLIANCE.”

FIELD OF THE INVENTION

[0002] This application relates to hard disc drives and moreparticularly to an apparatus and method for coupling a disc to a hub, soas to dampen radial vibrations of the disc.

BACKGROUND OF THE INVENTION

[0003] The storage medium for a disc drive is a flat, circular disccapable of retaining localized magnetic fields. The data that are storedupon the disc find physical representation through these localizedmagnetic fields. The data are arranged on the disc in concentric,circular paths known as tracks.

[0004] The localized magnetic fields can be detected by a magneticallysensitive head when they are brought in close proximity to the head.During operation the disc continually rotates, meaning that for eachrotation, a head fixed a given radius from the center of the disc wouldencounter every localized magnetic field along a given track. Alteringthe radial coordinate of the head allows the head to read or write dataalong a different track.

[0005] The head is mounted upon an actuator arm that is rotated by aservo control system. Accordingly, the track position of the head iscontrolled by the servo system. During a read or write operation, forexample, the servo control system ensures that the head remains orientedover the proper track, a function known as “track following.” The moreaccurately track following can be performed, the closer together thevarious tracks on the disc may be spaced, resulting in greaterinformation density. Increased accuracy in track following also permitsa higher rotational speed for the disc, resulting in faster access toinformation stored thereon.

[0006] One factor that renders track following less precise is thepresence of radial vibrations. Radial vibrations cause the disc to moverelative to the head, thereby moving the head away from the center ofthe particular track it is following. Radial vibrations may result fromimperfections in the bearings interposed between the rotating andstationary members of the spin motor in the disc drive. Theseimperfections cause the rotating member (the hub) to vibrate radially asit rotates; in conventional disc drives, the disc is rigidly mounted tothe hub, meaning that vibration of the hub results in vibration of thedisc. Generally, the faster the disk rotates, the more pronounced theradial vibrations. Other sources of radial vibrations include windageeffects that act upon the disc as it rotates and accident jarring of thedisc drive itself.

[0007] If radial vibrations of the disc could be attenuated, trackfollowing could be performed with greater precision. Benefits, such asincreased information density and increased rotational speed of thedisc, may be realized by such attenuation. Accordingly, there is a needfor an apparatus or method for reducing radial vibrations within discdrives.

SUMMARY OF THE INVENTION

[0008] Against this backdrop the present invention has been developed.The apparatus includes a hub that rotates, which is coupled to amagnetically encodable disc via a flexible joint. The flexible joint maybe made of a viscoelastic adhesive. There exists no rigid couplingbetween the disc and the hub, meaning that radial vibrations of the hubresult in compression or decompression of the flexible joint, ratherthan in translation of the disc.

[0009] Another embodiment of the present invention involves applying aviscoelastic adhesive to a hub, and applying a disc to the viscoelasticadhesive without otherwise rigidly coupling the disc to the hub.

[0010] These and various other features as well as advantages whichcharacterize the present invention will be apparent from a reading ofthe following detailed description and a review of the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a plan view of a disc drive incorporating an embodimentof the present invention showing the primary internal components.

[0012]FIG. 2 depicts a cross-sectional view of an embodiment of adisc-and-hub assembly in accordance with the present invention.

[0013]FIG. 3 depicts a cross-sectional view of another embodiment of adisc-and-hub assembly in accordance with the present invention.

[0014]FIG. 4 depicts a cross-sectional view of yet another embodiment ofa disc-and-hub assembly in accordance with the present invention.

DETAILED DESCRIPTION

[0015] In traditional disc drives, one or more discs are rigidly coupled(either directly or via one or more intermediate rigid members) to thehub. The consequence of this rigid form of coupling is that radialvibration of the hub results in radial vibration of the disc—a resultthat is inimical to the goal of precise track following.

[0016] A disc may be protected from radial vibrations by flexiblycoupling the disc to the hub. For example, a viscoelastic adhesive maybe used to couple the disc to the hub. Under such a flexible couplingscheme, radial vibration of the hub results in radial compression ordecompression of the viscoelastic adhesive, rather than in radial motionof the disc adhered thereto. To achieve this result, rigid coupling ofthe disc and hub should be entirely absent.

[0017] In the discussion that follows, the passages associated with FIG.1 are intended generally to familiarize the reader with the operation ofa disc drive and briefly introduce the invention. The passagesassociated with FIGS. 2-4 focus more particularly on flexible couplingof a disc and hub.

[0018] A disc drive 100 constructed in accordance with a preferredembodiment of the present invention is shown in FIG. 1. The disc drive100 includes a base 102 to which various components of the disc drive100 are mounted. A top cover 104, shown partially cut away, cooperateswith the base 102 to form an internal, sealed environment for the discdrive in a conventional manner. The components include a spindle motor(also known as a hub) 106 which rotates one or more discs 108 at aconstant high speed. The hub 106 and the one or more discs 108 arecoupled flexibly, so as to dampen radial vibrations. FIGS. 2-4 depictvarious embodiments of flexible coupling of a disc 108 to a hub 106.

[0019] Information is written to and read from tracks on the discs 108through the use of an actuator assembly 110, which rotates during a seekoperation about a bearing shaft assembly 112 positioned adjacent thediscs 108. The actuator assembly 110 includes one or more actuator arms114 which extend towards the discs 108, with one or more flexures 116extending from each of the actuator arms 114. Mounted at the distal endof each of the flexures 116 is a head 118 which includes an air bearingslider enabling the head 118 to fly in close proximity above thecorresponding surface of the associated disc 108.

[0020] During a seek operation, the track position of the heads 118 iscontrolled through the use of a voice coil motor (VCM) 124, whichtypically includes a coil 126 attached to the actuator assembly 110, aswell as one or more permanent magnets 128 which establish a magneticfield in which the coil 126 is immersed. The controlled application ofcurrent to the coil 126 causes magnetic interaction between thepermanent magnets 128 and the coil 126 so that the coil 126 moves inaccordance with the well known Lorentz relationship. As the coil 126moves, the actuator assembly 110 pivots about the bearing shaft assembly112, and the heads 118 are caused to move across the surfaces of the oneor more discs 108.

[0021] The spindle motor 116 is typically de-energized when the discdrive 100 is not in use for extended periods of time. The heads 118 aremoved over park zones 120 near the inner diameter of the discs 108 whenthe drive motor is de-energized. The heads 118 are secured over the parkzones 120 through the use of an actuator latch arrangement, whichprevents inadvertent rotation of the actuator assembly 110 when theheads are parked. Some disk drives do not use park zones. In these diskdrives, the heads are unloaded onto a ramp adjacent the disk and loadedfrom a ramp adjacent the disk.

[0022] A flex assembly 130 provides the requisite electrical connectionpaths for the actuator assembly 110 while allowing pivotal movement ofthe actuator assembly 110 during operation. The flex assembly includes aprinted circuit board 132 to which head wires (not shown) are connected;the head wires being routed along the actuator arms 114 and the flexures116 to the heads 118. The printed circuit board 132 typically includescircuitry for controlling the write currents applied to the heads 118during a write operation and a preamplifier for amplifying read signalsgenerated by the heads 118 during a read operation. The flex assemblyterminates at a flex bracket 134 for communication through the base deck102 to a disc drive printed circuit board (not shown) mounted to thebottom side of the disc drive 100.

[0023]FIG. 2 depicts a cross-sectional view of an embodiment of adisc-and-hub assembly 200 in accordance with the present invention. Thedisc-and-hub assembly 200 is composed of a hub 202 that has a radiallyprotruding hub flange 204, a flexible joint 206, and a magneticallyencodable disc 208 that is coupled to the hub 202 via the flexible joint206. There exists no rigid coupling, either direct or indirect, of thedisc 208 and the hub 202. The disc-and-hub assembly 200 rotates about anaxis 201 orthogonal to the disc 208. The disc 208 has an innercircumferential edge 210 and an outer circumferential edge 212.Likewise, the flexible joint 206 has an inner edge 214 and an outer edge216.

[0024] As the disc-and-hub assembly 200 spins about its axis 201, thehub 202 vibrates radially. Radial vibrations result from imperfectionsin the bearings interposed between the hub 202 and stationary members ofthe motor in the disc drive. These imperfections cause the hub 202 tovibrate radially as it rotates. Generally, the faster the disc-and-hubassembly 200 rotates, the more pronounced the radial vibrations. In adisc drive that rotates at 60 Hertz (Hz), radial vibrations arepronounced in a frequency band of approximately 41-1800 Hz. In thedisc-and-hub assembly of FIG. 2, radial vibration of the hub 202 causesthe flange 204 to vibrate radially as well (because the hub 202 andflange 204 are a single, rigid structure). However, as the flange 204vibrates radially, the flexible joint 206 compresses or decompressradially, rather than reacting to the vibration by translating, so thatthe disc 208 may remain untranslated by the radial vibration.

[0025] The flexible joint 206 may be composed of a viscoelasticadhesive. In one embodiment, the viscoelastic adhesive is formed as anadhesive sheet. One example of such a viscoelastic adhesive sheet is“Ultra-pure Viscoelastic Damping Polymer 242F02,” available from 3MCorporation. If formed into a sheet, the viscoelastic adhesive material206 may be flanked by a protective lining to keep the adhesive 206 cleanprior to adhesion to the hub 202 or flange 204. The viscoelasticmaterial may be applied to a surface of the hub 202 or hub flange 204 bypeeling off one of the protective layers from the viscoelastic adhesivesheet, and then attaching one edge of the sheet to the hub 202 or flange204; thereafter, the remainder of the sheet is gradually lowered(sometimes at an acute angle) on to the hub 202 or flange 204. To applythe disc 208 to the viscoelastic adhesive 206, the remaining protectivelayer is peeled off, and the disc 208 is placed upon the adhesive 206with even pressure to ensure adhesion.

[0026] Although the disclosure has heretofore focussed upon radialvibrations, axial vibrations are also detrimental to track following. Asthe disc 208 bends (due to its rotation), axial vibrations result inradial displacment, just as radial vibrations result in radialdisplacement. As stated earlier, the consequence of radial displacementis unwanted relative motion between the disc 208 and the read/write head(not depicted in FIG. 2). The viscoelastic joint 206 also dampens axialvibrations by responding to them by compressing or decompressingaxially, rather than translating the disc 208 in an axial direction.

[0027] Axial vibrations grow in intensity as they propagate toward theouter circumferential edge 212 of the disc 208. Thus, for the sake ofdamping axial vibrations where they are relatively intense, it isdesirable to adhere the viscoelastic adhesive 206 to a surface that istoward the outer circumferential edge of the disc 208. In oneembodiment, the outer edge 216 of the viscoelastic material 206 is flushwith the outer circumferential edge 212 of the disc 208.

[0028] With regard to the geometry of the viscoelastic adhesive 206, onefactor to be taken into account is the compliance of the viscoelasticadhesive 206. The compliance of a viscoelastic adhesive 206 isproportional to its height. Accordingly, for the sake of compliantcoupling of the disk 208 to the hub 202, it is desirable for theviscoelastic adhesive 206 to be relatively tall. In one embodiment, theviscoelastic adhesive 206 may be at least 0.001 inches in height (asmeasured between flange 204 surface and disk 208 surface). In otherembodiments, the viscoelastic adhesive 206 may be at least 0.005 inchesin height.

[0029] Another factor influencing the geometry of the viscoelasticadhesive 206 is its capacity to dissipate energy. Smaller volumes ofviscoelastic adhesive 206 dissipate greater amounts of energy. Thus, tomaximally damp vibrations (be they radial or axial), the viscoelasticadhesive 206 should be relatively tall (so as to be compliant) andrelatively thin (so as to have a small volume, permitting it todissipate the energy transmitted by radial and axial vibrations). On theother hand, since the contact area between the viscoelastic adhesive 206and the disc 208 is responsible for coupling the disc 208 to the hub202, there must be a sufficient contact area to prevent the disc 208from literally falling off of the hub 202 during rotation. To balancethese considerations, some embodiments may have the inner edge 214 ofthe viscoelastic adhesive 206 as close as 2 millimeters to the outeredge 216 of the viscoelastic adhesive 206. In other embodiments, thevolume of the viscoelastic material 206 may be between 0.5% and 5% ofthat of the disc 208. In still other embodiments, between 5% and 100% ofthe surface area of the disc 208 may be adhered to the viscoelasticadhesive 206.

[0030]FIG. 3 depicts a cross-sectional view of another embodiment of adisc-and-hub assembly 300 in accordance with the present invention. Thedisc-and-hub assembly 300 is composed of a hub 302 that has a radiallyprotruding hub flange 304, a flexible joint 306, and a magneticallyencodable disc 308 that is coupled to the hub 302 via the flexible joint306. There exists no rigid coupling, either direct or indirect, of thedisc 308 and the hub 302. The disc-and-hub assembly 300 rotates about anaxis 301 orthogonal to the disc 308.

[0031] Once again, the flexible joint 306 may be composed of aviscoelastic adhesive. Thus, the physical properties regardingcompliance and energy dissipation, which were discussed with referenceto FIG. 2, apply equally to the viscoelastic material 306 depicted inFIG. 3. Thus, there are various embodiments of the disc-and-hub assembly300 depicted in FIG. 3 in which the viscoelastic material 306 takes onthe geometric characteristics described in the passages relating to FIG.2.

[0032] In the disc-and-hub assembly 300 of FIG. 3, the hub flange 304 iscomposed of an upper plane 310 and a lower plane 312. The disc 308 restsatop the upper plane 310 and also rests atop a layer of viscoelasticmaterial 306 filling at least a portion of the gap between the lowerplane 312 and the disc 308. The upper plane 310 is used to achieveprecise z-axis registration of the disc 308. Although the disc restsatop the upper plane 310, there exists no rigid coupling between thedisc 308 and the upper plane 310. Stated another way, the disc 308 maymove relative to the upper plane 310, effectively limited only by itsjoinder with the viscoelastic material 306. The difference in heightbetween the upper plane 310 and the lower plane 312 defines the heightof the viscoelastic material 306. In one embodiment, the difference inheight between the two planes 310, 312 is at least approximately 0.0005inches. In another embodiment, the difference is at least approximately0.005 inches.

[0033]FIG. 4 depicts a cross-sectional view of yet another embodiment ofa disc-and-hub assembly 400 in accordance with the present invention.The disc-and-hub assembly 400 is composed of a hub 402, a flexible joint406, and a magnetically encodable disc 408 that is coupled to the hub402 via the flexible joint 406. There exists no rigid coupling, eitherdirect or indirect, of the disc 408 and the hub 402. The disc-and-hubassembly 400 rotates about an axis 401 orthogonal to the disc 408.

[0034] Once again, the flexible joint 406 may be composed of aviscoelastic adhesive. Thus, the physical properties regardingcompliance and energy dissipation, which were discussed with referenceto FIG. 2, apply equally to the viscoelastic material 406 depicted inFIG. 4. Therefore, there are various embodiments of the disc-and-hubassembly 400 depicted in FIG. 4 in which the viscoelastic material 406takes on the geometric characteristics described in the passagesrelating to FIG. 2.

[0035] In the disc-and-hub assembly 400 of FIG. 4, the disc 408 isadhered to a top surface of the hub 402, thus eliminating the need for ahub flange. Such an arrangement permits the disc-and-hub assembly 400 toachieve a low profile, thereby permitting the disc drive in which theassembly 400 is used to also have a low profile.

[0036] To summarize preferred embodiments of the present invention, anapparatus for for coupling a disc to a hub, so as to dampen radialvibrations of the disc consists of the following. A hub (such as 202)that rotates is coupled to a magnetically encodable disc (such as 208)via a viscoelastic joint (such as 206). There exists no rigid couplingbetween the disc (such as 208) and the hub (such as 202). The hub (suchas 202) may be shaped to possess a flange (such as 204), in which casethe viscoelastic adhesive (such as 206) is disposed upon the flange(such as 204), and the disc (such as 208) is disposed upon theviscoelastic adhesive (such as 206). Optionally, the flange (such as304) may be composed of an upper plane (such as 310) and a lower plane(such as 312), in which case the disc rests upon the upper plane (suchas 310), and a viscoelastic adhesive layer (such as 306) is disposedupon the lower plane (such as 312), thereby adhering the disc (such as308) to the lower plane (such as 312). Optionally, the upper plane (suchas 310) and the lower plane (such as 312) may be separated by at least0.0005 inches, resulting in a viscoaelastic layer (such as 306) at least0.0005 inches in height. In another embodiment, the upper plane (such as310) and the lower plane (such as 312) may be separated by at least0.005 inches. In yet another ambodiment, between 5% and 100% of thesurface area of the disc (such as 208) is adhered to the viscoelasticlayer (such as 206). In yet another embodiment, the viscoelasticadhesive layer (such as 206) has a volume of between 0.5% and 5% of thatof the disc (such as 208). The viscoelastic adhesive layer (such as 206)is characterized by an inner edge (such as 214) and an outer edge (suchas 216), and the disc (such as 208) is characterized by an outercircumferential edge (such as 212) and an inner circumferential edge(such as 210). In one embodiment, the outer edge (such as 216) of theviscoelastic adhesive layer is flush with the outer circumfrential edgeof the disc (such as 212). In another embodiment, the inner edge (suchas 214) of the viscoelastic adhesive layer is within 2 millimeters ofthe outer circumferential edge of the disc (such as 212).

[0037] The disc (such as 208) may be adhered to the hub (such as 202) byapplying a viscoelastic adhesive (such as 206) to the hub (such as 202or 204), and applying the disc (such as 208) to the viscoelasticadhesive (such as 206) without otherwise rigidly coupling the disc (suchas 208) to the hub (such as 202 or 204). Optionally, the viscoelasticadhesive (such as 206) may be a viscoelastic adhesive tape. In thatcase, to attach the tape (such as 206) to the hub, a first protectivelayer is peeled from the viscoelastic adhesive tape (such as 206). Next,a first edge of the viscoelastic adhesive tape (such as 206) is attachedto the hub (such as 202 or 204). Then, the remainder of the viscoelasticadhesive tape (such as 206) is lowered at an acute angle on to the hub(such as 202 or 204). Finally, pressure is applied to the tape (such as206). To attach the disc (such as 208) to the viscoelastic adhesive tape(such as 206), first a protective layer is peeled from the viscoelasticadhesive tape (such as 206). Next, the disc (such as 208) is placed uponthe viscoelastic adhesive tape (such as 206). Finally, pressure isapplied to the disc (such as 208), so as to adhere the disc (such as208) to the viscoelastic adhesive tape (such as 206).

[0038] According to another embodiment of the inventoin, a disc-and-hubassembly that dampens radial vibrations is composed of a hub (such as202) that rotates, thereby rotating a magnetically encodable disc, and ameans (such as 206) for flexibly coupling the disc (such as 208) to thehub (such as 202) without rigidly coupling the disc (such as 208) andthe hub (such as 202). The means (such as 206) for flexibly coupling thedisc to the hub may be disposed atop the hub (such as 202), and the disc(such as 208) may disposed atop the means (such as 206) for flexiblycoupling the disc (such as 208) to the hub (such as 202).

[0039] It will be clear that the present invention is well adapted toattain the ends and advantages mentioned as well as those inherenttherein. While a presently preferred embodiment has been described forpurposes of this disclosure, various changes and modifications may bemade which are well within the scope of the present invention. Forexample, the hub flange may be of another shape or may consist of morethan two planes. Additionally, the flexible joint may consist of anothersuitable material other than a viscoelastic material. Numerous otherchanges may be made which will readily suggest themselves to thoseskilled in the art and which are encompassed in the spirit of theinvention disclosed and as defined in the appended claims.

What is claimed is:
 1. A disc drive with a disc-and-hub assembly thatdampens radial vibrations, the disc-and-hub assembly comprising: a hubthat rotates; a viscoelastic joint that couples a magnetically encodabledisc to the hub, thereby causing the disc to rotate with the hub; and anabsence of rigid coupling between the disc and the hub.
 2. The discdrive of claim 1, wherein: the hub possesses a flange; a viscoelasticadhesive layer is disposed upon the flange; and the disc is disposedupon the viscoelastic adhesive.
 3. The disc drive of claim 2, wherein:the flange possesses a surface comprised of an upper plane and a lowerplane; the disc rests upon the upper plane; and the viscoelasticadhesive layer is disposed upon the lower plane, so that the disc isadhered to the lower plane by the viscoelastic adhesive.
 4. The discdrive of claim 3, wherein the upper plane and the lower plane areseparated by at least 0.0005 inches.
 5. The disc drive of claim 4,wherein the upper plane and the lower plane are separated by at least0.005 inches.
 6. The disc drive of claim 2, wherein the viscoelasticadhesive layer is at least 0.001 inches in height.
 7. The disc drive ofclaim 2, wherein between 5% and 100% of the surface area of the disc isadhered to the viscoelastic layer.
 8. The disc of claim 2, wherein: theviscoelastic adhesive layer possesses an inner edge and an outer edge;the disc possesses an outer circumferential edge and an innercircumferential edge; and the outer edge of the viscoelastic adhesivelayer is flush with the outer circumfrential edge of the disc.
 9. Thedisc drive of claim 2, wherein: the viscoelastic adhesive layer ischaracterized by an inner edge and an outer edge; the disc ischaracterized by an outer circumferential edge and an innercircumferential edge; and the inner edge of the viscoelastic adhesivelayer is within 2 millimeters of the outer circumferential edge of thedisc.
 10. The disc drive of claim 2, wherein the viscoelastic adhesivelayer has a volume of between 0.5% and 5% of that of the disc.
 11. Thedisc drive of claim 1, wherein the viscoelastic adhesive layer has avolume of between 0.5% and 5% of that of the disc.
 12. A method ofcoupling a disc to a hub, so as to dampen radial vibration of the disc,the method comprising: applying a viscoelastic adhesive to the hub; andapplying the disc to the viscoelastic adhesive without otherwise rigidlycoupling the disc to the hub.
 13. The method of claim 12, wherein theviscoelastic adhesive comprises a viscoelastic adhesive tape, andwherein applying the viscoelastic adhesive tape to the hub comprises:peeling off a first protective layer from the viscoelastic adhesivetape; attaching a first edge of the viscoelastic adhesive tape to thehub; applying the viscoelastic adhesive tape, at an acute angle, to thehub; and applying pressure to the tape.
 14. The method of claim 12,wherein applying the viscoelastic adhesive to the hub comprises applyingthe viscoelastic adhesive to the hub throughout a surface area that isbetween 5% and 100% of the surface of the disc.
 15. The method of claim12, wherein applying the viscoelastic adhesive to the hub comprisesapplying the viscoelastic adhesive to a hub flange protruding from thehub.
 16. The method of claim 12, wherein applying the viscoelasticadhesive to the hub comprises applying a volume of viscoelastic adhesiveto the hub that is between 0.5% and 5% of the volume of the disc. 17.The method of claim 12, wherein the viscoelastic adhesive comprises aviscoelastic adhesive tape, and wherein applying the disc to theviscoelastic adhesive tape comprises: peeling off a protective layerfrom the viscoelastic adhesive tape; placing the disc upon theviscoelastic adhesive tape; and applying pressure to the disc, so as toadhere the disc to the viscoelastic adhesive tape.
 18. A disc drive witha disc-and-hub assembly that dampens radial vibrations, the disc-and-hubassembly comprising: a hub that rotates, thereby rotating a magneticallyencodable disc; and a means for flexibly coupling the disc to the hubwithout rigidly coupling the disc and the hub.
 19. The disc drive ofclaim 18, wherein the means for flexibly coupling the disc to the hubcomprises a viscoelastic adhesive.
 20. The disc drive of claim 19,wherein the viscoelastic adhesive has a surface area of between 5% and100% of the surface area of the disc.
 21. The disc drive of claim 19,wherein the viscoelastic adhesive has a volume of between 0.5% and 5% ofthat of the disc.
 22. The disc drive of claim 18, wherein: the means forflexibly coupling the disc to the hub is disposed atop the hub; and thedisc is disposed atop the means for flexibly coupling the disc to thehub.